The present invention generally relates to apparatus and method for plasma processing. Specifically, the present invention relates to a plasma processing apparatus including a reaction chamber where plasma is generated from a reactive gas introduced thereto and a film on a substrate is processed with the plasma generated, and to a plasma processing method for performing plasma etching in the reaction chamber.
Plasma processing methods are applied to various kinds of processing steps like etching a film on a substrate with plasma to form a fine-line pattern, and play a significant role in the fabrication process of a semiconductor device.
The main members (e.g., the walls) of a reaction chamber for performing various types of plasma processing such as plasma etching are ordinarily made of aluminum. However, aluminum acts as an acceptor (i.e., p-type impurity) against silicon, which is a typical material of a semiconductor substrate. Accordingly, it is necessary to prevent aluminum, coming from an etched part of a main member of the reaction chamber such as a wall thereof, from diffusing on the semiconductor substrate. Thus, the main members of the reaction chamber are usually covered with protective members lest the main members should be etched by plasma.
A protective member is ordinarily made of alumina (aluminum oxide) obtained by subjecting the surface of aluminum to an anodic oxide coating treatment (i.e., a so-called "ALUMITE " treatment), because this treatment is easy. In general, alumina is hard to be etched. Accordingly, if a protective member, made of aluminum including alumina regions on the surface thereof, is used, then impurities generated from the protective member can be reduced.
Recently, such protective members, covering the main members of a reaction chamber, not only plays the originally intended role of protecting the main members from plasma, but also is positively used for controlling the etching characteristics. Its objective is to properly deal with the everlasting miniaturization of semiconductor integrated circuits. Also, since the density of plasma has been drastically increased nowadays, the state of plasma affects the etching characteristics to a far larger degree than what it used to be. In view of these situations, the state of plasma is desirably kept as uniform and constant as possible. For example, suppose a silicon dioxide film on a semiconductor substrate is etched with plasma. In such a case, if the main members of a reaction chamber are covered with protective members made of quartz, which has the same composition as the silicon dioxide film, then a region where plasma has been generated is surrounded by silicon dioxide (or quartz). Accordingly, the plasma generated in the region equally etches the silicon dioxide film on the semiconductor substrate and the protective members of the reaction chamber under the same conditions. As a result, the state of the plasma is invariable with the passage of time and spatially uniform.
Nevertheless, if the main members of a reaction chamber are covered with such protective members made of quartz, then the quartz protective members are also etched unintentionally as the etching on the silicon dioxide film proceeds. However, the substance resulting from the etched quartz of the protective member acts as neither acceptor nor donor against silicon unlike aluminum. Accordingly, even if such a substance diffuses onto the semiconductor substrate, the electrical characteristics of a semiconductor device are not adversely affected by the substance.
Because of these reasons, quartz is superior to aluminum as a material for the protective members of the main members of a reaction chamber used for performing plasma etching on a semiconductor integrated circuit component, which should be formed in an even smaller size.
Also, according to a proposed technique, alumina and quartz are used in combination. Specifically, alumina is used for protective members covering the main members of a reaction chamber, because alumina has the above-described advantages. And a ring member made of quartz is disposed around a substrate placed on a sample stage, because the etching characteristics are particularly affected by the state of plasma on the periphery of a film to be processed. The objective thereof will be described below. If a silicon dioxide film, or the film to be processed, and the member disposed around the substrate on the sample stage are made of different materials, then the objects in contact with the plasma are greatly different from each other in the vicinity of the interface between the periphery of the silicon dioxide film and the sample stage. Accordingly, the state of the plasma on the periphery of the silicon dioxide film is different from the state of the plasma at the center of the silicon dioxide film. As a result, the characteristics of etching on the silicon dioxide film are variable within the plane of the silicon dioxide film. Therefore, in order to keep the state of plasma constant, the quartz ring member is disposed around the substrate placed on the sample stage.
If the main members of a reaction chamber are covered with quartz protective members or if a quartz ring member is disposed around a substrate placed on a sample stage, then the state of plasma can be surely kept constant and uniform. However, even in such a case, it is still impossible to prevent impurities, deposited on a film to be processed, from adversely affecting the electrical characteristics of a semiconductor device. This problem will be briefly discussed below.
For example, if impurities are deposited on a film to be processed, then a pause time (i.e., a charge retention time per memory cell), one of the parameters for evaluating the reliability of a DRAM, disadvantageously decreases. It is considered that the pause time probably decreases because of the generation of leakage current resulting from the deposition of impurities such as sodium onto a memory cell of the DRAM.
Thus, it has been believed an urgent task to prevent impurities, contained in reactants flaked off the main or protective members of a reaction chamber, from being deposited on a film to be processed by regularly cleaning the main and protective members and removing the deposited reactants therefrom.
In order to confirm the effects of such regular cleaning, the present inventors carried out experimental plasma etching with a reduced amount of reactants deposited on the main and protective members of the reaction chamber by cleaning these members regularly and carefully. However, we faced a problem that the pause time of a DRAM still could not be increased.